1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) for driving pixels by a thin film transistor (TFT) to perform liquid crystal display operation, and, more particularly, to a reflection type liquid crystal display utilizing poly silicon which is prepared using in a low-temperature process and to a projector employing this reflection type liquid crystal display.
2. Description of the Related Art
A liquid crystal display having a liquid crystal sealed between a pair of substrates for applying a voltage to the liquid crystal to perform desirable display operation has an advantage that it is small-sized, thin, and has reduced power consumption. For these reasons, liquid crystal displays are now used in a variety of OA and AV devices, portable or on-board information devices, and the like. It has also been proposed a transmission type liquid crystal display be used as a projector.
An active-matrix liquid crystal display using a thin film transistor (hereinafter referred to as a TFT) as a switching element for driving each liquid crystal pixel can selectively drive each TFT to select a particular liquid crystal pixel. Therefore, image display with no cross talk and with a higher resolution can be obtained.
Two known TFTs used in liquid crystal displays are amorphous silicon TFTs using amorphous silicon for an active layer and a poly silicon TFT using more highly mobile poly silicon for the active layer. The amorphous silicon TFT is most common in large-sized displays and the like because it can be formed over a large area through a low-temperature process. On the other hand, the poly silicon TFT has a higher mobility than the amorphous silicon and can form an element in a self-aligned manner. Therefore, the poly silicon TFT can more easily reduce TFT and pixel area than can the amorphous silicon TFT, so that a display having a higher resolution can more easily be manufactured. With the use of the poly silicon, the TFT can easily have a CMOS structure. For this reason, a driver TFT for driving a display section TFT can be formed on the same substrate at almost the same processes as the display section TFT.
As stated above, the poly silicon TFT has attractive characteristics and can include a driver on its substrate. The poly silicon is formed by polycrystallizing amorphous silicon in a high-temperature process. As the amorphous silicon is exposed to a high temperature during the process, an inexpensive glass substrate cannot be used for the substrate. For this reason, the poly silicon TFT faces a practical problem with respect to cost.
However, polycrystallization technology using an annealing treatment such as laser annealing, lamp annealing, or the like has been improved. Consequently, it has become possible to manufacture poly silicon in the low-temperature (below 600.degree. C.) process. By forming a poly silicon TFT in a low-temperature process, an inexpensive glass substrate can be developed for use as a TFT substrate. Cost can be reduced and, furthermore, effective area can be increased. As a result, a poly silicon TFT (hereinafter referred to as a low-temperature poly silicon TFT) formed in a low-temperature process has practical uses.
Although low-temperature poly silicon TFTs have been put into practical use, a liquid crystal material and a panel structure which are optimum for exhibiting the characteristics of the low-temperature poly silicon TFT for a liquid crystal display at the maximum and for enhancing the characteristics have yet to be developed. Commonly, a material and a structure which had been used in a conventional amorphous silicon TFT liquid crystal display are employed without adaptation. Consequently, there is a problem that the characteristics of the poly silicon TFT cannot be exhibited fully.
As mentioned above, a liquid crystal display can be utilized in a projector. More specifically, light passing or reflected through the liquid crystal display is controlled by the liquid crystal display, and is projected onto a screen. Consequently, an image is displayed on that screen. With the use of a reflection type liquid crystal display, the size of the projector can be comparatively reduced. In an ordinary liquid crystal display, however, a viewing angle is small. Therefore, there has been a problem that it is difficult to obtain sufficient reflected light.